A very recent introduction, first reported in the UK in 2010 and
since having been found in three different locations. This is currently a high
priority for biosecurity, the emphasis of management being on stopping any
further spread. An alert species, it must be reported as soon as possible on
being discovered. The killer shrimp has been ranked among the ‘Top 100’
invasive alien species in Europe by an EC
initiative. In Britain
the Environment Agency carry out extensive monitoring of water bodies for this
The risk assessment for killer shrimp has scored this
species ‘very high risk’ for impact and ‘high’ risk overall. It is a voracious
predator of native shrimp, other invertebrates, eggs and small fish. Through
direct predation and indirect trophic effects, it has the potential to
significantly alter ecosystems.
You can help stop the spread of killer shrimp and other
aquatic invasives by following three simple steps whenever you visit
freshwaters: Check – Clean – Dry.
For more biosecurity advice, please visit www.nonnativespecies.org/alerts/killershrimp
Zebra musselDreissena polymorpha
Like killer shrimp, the zebra mussel is an import from the Ponto-Caspian region of Western Asia. However it has been in Britain much longer, since the early 19th century. It is present in Lancashire but not widespread. Zebra mussels are so named because of the distinctive zigzag banding on their shells. They out-compete native fauna, forming dense colonies on the substrate under still or flowing water. They also filter large quantities of water, removing planktonic animals which are the food of many other species.
Chinese mitten crab Eriocheir sinensis
This large riverine crab gets its name from the velvety hair that covers its pincers. Transported to Britain in ballast water, the mitten crab was first recorded in the river Thames. It has now been found in several locations includingSouth Cumbria. This is an alert species so please report immediately if seen. Mitten crabs are voracious predators, eating a variety of native wildlife. They also riddle the river bank with burrows, causing erosion and subsidence.
Canadian and Nuttall’s pondweeds
These pondweeds are true aquatic plants which grow in deep,
slow flowing water. Dispersal is by vegetative means – small fronds of plant
break off and float away to settle elsewhere. The impact of these plants is not
well known but they can form dense beds which exclude other aquatic plants and
may impede stream flow in some circumstances.
Curly water thyme
This waterweed superficially resembles the Elodea species (Canadian and Nuttall’s pondweeds), however it is more invasive. It is native to Southern Africa and inhabits deep, still or slow moving water. All plants in Britain are female and it is spread by the movement of plant fragments. Like other troublesome aquatic invasives, this species impacts on waterways by growing dense colonies and excluding native plants.
This plant is a recent introduction from the Americas.
It spreads vigorously across water and damp soil, preventing light reaching the
water and excluding other plants. It also interferes with recreational use of
the water. At the moment, floating pennywort is not widespread in the North West. It is
important to monitor this plant closely to prevent further spread.
Creeping water primrose Ludwigia peploides
This is a worrisome plant that is fortunately not yet present in Lancashire. It is native throughout the Americas and was first recorded in Britain in 1999. This is an alert species which must be reported as soon as possible to the Non-Native Species Secretariat. Creeping water primrose forms large monoculture stands, shading out other plants, disrupting water flow and increasing the flood risk. It has an incredible potential for growth and can double its biomass in 20 days.
Named after its feathery appearance, this plant originates
from South America. The plant grows submerged
in the water and may crowd out other aquatic plants due to its vigorous habit. Its
habitat tends to be still water: ponds, ditches and canals, but it is occasionally
found in slow flowing rivers. Like floating pennywort, parrot’s feather is not
well established in Lancashire.
Azolla is the only floating fern found in Britain. It is often spread
inadvertently on aquatic garden plants and is sometimes even sold as an
ornamental. From just a small fragment, the fern will quickly spread across the
surface of the water, forming a dense mat and completely excluding light
penetration. The roots of the plant form a symbiosis with a nitrogen fixing
cyanobacteria, enabling azolla to grow in nutrient poor water.
Australian swamp stonecrop
Also known as New Zealand pigmyweed, this
succulent perennial is one of the more problematic of the invasive aquatic
plants. Its habitat is ponds, canals, wetlands and other areas of still water.
It has a creeping habit and covers the surface of the water to a depth of half
a metre. Dispersal in Britain
is primarily vegetative; stonecrop can regenerate from a single node on a 10mm